Synapses and dendritic spines are important in learning and memory. Indeed, synapse loss is thought to be a main contributor to the cognitive decline seen in Alzheimer's disease (AD). Synapse functions are affected by amyloid precursor protein (APP) and Reelin, a ligand for APP and the apoE receptor VLDLR, both of which also are involved in AD pathology. Recently, we and others found that both APP and Reelin are important for dendritic spine formation. Additionally, we indentified a novel interaction between APP and the very low density lipoprotein receptor (VLDLR), an apoE receptor that plays an important role in cognitive performance as well as LTP. However, whether VLDLR can alters dendritic spine structure and whether an APP and VLDLR complex can have synergistic or negative effects on dendritic spine formation is unknown. To address this, we initially examined the effects of VLDLR on dendritic spine formation and found that overexpression of VLDLR increased spine density, and knockdown of endogenous VLDLR decreased spine density in primary hippocampal neurons. Also, VLDLR-infected neurons increased the activity of Ras, which is known to promote dendritic spine formation. Moreover, we found that extracellular domain of VLDLR is required for dendritic spine formation, suggesting that VLDLR alters dendritic spine density through extracellular ligand interactions. Based on the literature and our findings, we hypothesize that VLDLR promotes dendritic spine formation through association with Reelin and/or APP. In this application, we will examine whether VLDLR alters dendritic spine density in vivo, and will investigate the molecular mechanism by which VLDLR affects dendritic spine formation. Additionally, we will examine whether the interactions between VLDLR and APP work independently, competitively or synergistically to alter dendritic spine formation. We will further examine whether these effects are regulated by the extracellular ligand, Reelin, which interacts with both APP and VLDLR. We will utilize molecular and cell biology approaches to elucidate the molecular mechanisms by which VLDLR cooperates or competes with APP and/or Reelin to regulate dendritic spine formation. Therefore, in Aim 1, we will determine whether VLDLR regulates dendritic spine formation. In Aim 2, we will investigate whether VLDLR mediates dendritic spine formation through Reelin and/or APP. The proposed experiments will further our understanding of the physiological involvement of the interaction between VLDLR and APP and/or the extracellular ligand, Reelin, in dendritic spine formation. Our results will contribute to the growing evidence that these proteins, VLDLR and APP, have crucial functions in dendritic spine formation, and therefore, also in synapse connection and in learning and memory. The physiological mechanisms investigated in these experiments will advance the understanding of, and the development of therapeutics for the cognitive decline and synapse loss observed in Alzheimer's disease.